U.S. patent application number 12/086937 was filed with the patent office on 2010-04-29 for power plant with membrane water gas shift reactor system.
Invention is credited to Zissis Dardas, Mallika Gummalla, Benoit Olsommer, Ying She, Thomas Henry Vanderspurt.
Application Number | 20100104903 12/086937 |
Document ID | / |
Family ID | 38228643 |
Filed Date | 2010-04-29 |
United States Patent
Application |
20100104903 |
Kind Code |
A1 |
Gummalla; Mallika ; et
al. |
April 29, 2010 |
Power Plant With Membrane Water Gas Shift Reactor System
Abstract
The fuel processing system of the present invention supplies a
flow of H2-rich reformate to a water gas shift membrane reactor,
comprising a water gas shift reaction region and a permeate region,
separated by an H2-separation membrane H2 formed over a catalyst in
the reaction region selectively passes through the H2-separation
membrane to the permeate region for delivery to a use point (such
as the fuel cell of a fuel cell power plant) A sweep gas,
preferably steam, removes the H2 from the permeate region The
direction of sweep gas flow relative to the reformate flow is
controlled for H2-separation performance and is used to determine
the loading of the catalyst in the reaction region Coolant, thermal
and/or pressure control subsystems of the fuel cell power plant may
be integrated with the fuel processing system
Inventors: |
Gummalla; Mallika;
(Longmeadow, MA) ; Vanderspurt; Thomas Henry;
(Glastonbury, CT) ; She; Ying; (Worcester, MA)
; Dardas; Zissis; (Worcerter, MA) ; Olsommer;
Benoit; (South Glastonbury, CT) |
Correspondence
Address: |
Stephen A. Schneeberger
49 Arlington Road
West Hartford
CT
06107
US
|
Family ID: |
38228643 |
Appl. No.: |
12/086937 |
Filed: |
December 23, 2005 |
PCT Filed: |
December 23, 2005 |
PCT NO: |
PCT/US2005/047012 |
371 Date: |
June 20, 2008 |
Current U.S.
Class: |
429/420 |
Current CPC
Class: |
C01B 3/12 20130101; C01B
3/48 20130101; C01B 2203/066 20130101; H01M 2008/1095 20130101;
C01B 3/38 20130101; H01M 8/0668 20130101; Y02E 60/50 20130101; C01B
2203/0288 20130101; C01B 2203/041 20130101 |
Class at
Publication: |
429/20 |
International
Class: |
H01M 8/18 20060101
H01M008/18 |
Goverment Interests
[0001] The U.S. Government has a paid-up license in this invention
and the right in limited circumstances to require the patent owner
to license others on reasonable terms as provided for by the terms
of contract No. DE-FC26-05NT42453 awarded by the Department of
Energy.
Claims
1. A fuel cell power plant system (10) comprising: at least one
fuel cell stack assembly (12) including an anode (18), a cathode
(20), and a coolant channel (24); a fuel processing system (14,
114, 214) for providing H.sub.2 to the anode (18) and including a
water gas shift membrane reactor (62, 162)) having a reaction
region (74, 174) and a permeate region (76, 176) separated by a
H.sub.2 separation membrane (64, 164), the reaction region being
connected to receive a supply stream of H.sub.2-rich reformate (60)
and a supply of water (16e'', 73, 173) for supporting a water gas
shift reaction of the reformate to enhance the production of
H.sub.2 and to shift CO to CO.sub.2, the produced H.sub.2 being
selectively separated from the reformate stream via the membrane to
form a permeate (26, 126, 226) in the permeate region of the
reactor, and the reformate stream issuing from the reactor as a
retentate (66); a sweep gas (78, 178, 278) connected (80, 180, 280)
to flow through the permeate region of the water gas shift membrane
reactor; a source of heat (40, 70A/70B, 50, 62, 162); and a water
management system (16, 16', 16'', 116'', 12, 34) operatively
connected (30, 31, 32, 33) to the fuel cell assembly coolant
channel (24) for conducting water from and to the fuel cell
assembly, the water management system additionally being connected
(70A/70B, 52, 229) to the source of heat for converting some water
to steam, and the steam being operatively connected (80, 180, 280,)
to provide at least a portion of the sweep gas (78, 178, 278).
2. The fuel cell power plant system of claim 1 wherein the
reformate (60) flows through the reaction region of the water gas
shift membrane reactor in a first flow direction; and the stream of
sweep gas (78, 278) is connected to flow through the permeate
region in a second flow direction substantially counter to said
first flow direction.
3. The fuel cell power plant system of claim 2 wherein the reaction
region of the water gas shift membrane reactor includes an entry
portion (74A), an exit portion (74C), and an intermediate-portion
(74B) between the entry and exit portions relative to said first
flow direction of the reformate; and a water gas shift catalyst
(75) is loaded substantially only in the entry and the exit
portions of the water gas shift membrane reactor.
4. The fuel cell power plant system of claim 3 wherein the extent
of the entry portion (74A) and the extent of the exit portion (74B)
which each receive the loading of the catalyst (75) are each about
20% of the total extent of the reaction region of the water gas
shift membrane reactor.
5. The fuel cell power plant system of claim 3 including pressure
control means (53, 55) for regulating the operating pressure of at
least the reformate flow stream through the water gas shift
membrane reactor reaction region to be a moderate pressure in a
range of about 1 to 10 bar.
6. The fuel cell power plant system of claim 5 wherein the pressure
control means regulates the operating pressure of the reformate to
be about 6 to 7 bar.
7. The fuel cell power plant system of claim 5 wherein said
pressure control means comprises a compressor (53) for pressurizing
air to within said range of moderate pressure for delivery to at
least the water gas shift membrane reactor, an expander (55)
connected to receive said retentate (66) from said water gas shift
membrane reactor at a pressure substantially within said range of
moderate pressure and expanding said retentate to reduce the
pressure thereof and thereby release stored energy, and energy
utilization means (65, 67) operatively connected to the expander
and to the compressor for powering the compressor.
8. The fuel cell power plant system of claim 2 wherein said source
of heat comprises a burner (40), said burner being operatively
connected (66') to receive said retentate stream as a fuel
source.
9. The fuel cell power plant system of claim 1 wherein the
reformate (60) flows through the reaction region of the water gas
shift membrane reactor in a first flow direction; and the stream of
sweep gas (178) is connected to flow through the permeate region in
a second flow direction substantially the same as said first flow
direction.
10. The fuel cell power plant system of claim 9 wherein the
reaction region (174) of the water gas shift membrane reactor
includes an entry portion (174A), an exit portion (174C), and an
intermediate-portion (174B) between the entry and exit portions
relative to said first flow direction of the reformate; and a water
gas shift catalyst (75) is loaded substantially only in the entry
portion (174B) of the water gas shift membrane reactor.
11. The fuel cell power plant system of claim 10 wherein the entry
portion (174A) extends half the length of the reaction region of
the water gas shift membrane reactor, and the extent of the entry
portion that receives the water gas shift catalyst extends about
20% of the total extent of the reaction region of the water gas
shift membrane reactor.
12. The fuel cell power plant system of claim 1 further including
means (292, 228, 229) for providing a supply of inert gas (290),
said inert gas being operatively connected (280) in combination
with said steam to provide said sweep gas (278).
13. In a fuel processing system (14) for providing H.sub.2 from a
supply of H.sub.2-rich reformate (60), a water gas shift membrane
reactor (62, 162) having a reaction region (74, 174) and a permeate
region (76, 176) separated by an H.sub.2 separation membrane (64,
164), the reaction region containing a shift catalyst (75) and
being connected to receive a stream of the H.sub.2-rich reformate
(60) flowing there through in a first flow direction and a supply
of water (16e'', 73, 173) for supporting a water gas shift reaction
to enhance the production of H.sub.2 and to shift CO to CO.sub.2,
the produced H.sub.2 being selectively separated from the reformate
stream via the membrane to form a permeate (26, 126, 226) in the
permeate region of the reactor, a stream of sweep gas (78, 178,
278) connected to flow through the permeate region in a particular
second flow direction relative to the 1.sup.st flow direction of
the reformate, and wherein the catalyst in the reaction region is
distributed therein as a function of said 1.sup.st and 2.sup.nd
flow directions.
14. The fuel processing system of claim 13 wherein the reaction
region (74) of the water gas shift membrane reactor includes an
entry portion (74A), an exit portion (74C), and an intermediate
portion (74B) between the entry and exit portions relative to said
first flow direction of the reformate; the flow of the stream of
sweep gas through the permeate region in the second flow direction
is substantially counter to said first flow direction; and the
water gas shift catalyst is loaded substantially only in the entry
and the exit portions (74A and 74C) of the water gas shift membrane
reactor.
15. The fuel processing system of claim 13 wherein the reaction
region (174) of the water gas shift membrane reactor includes an
entry portion (174A), an exit portion (174C), and an intermediate
portion (174B) between the entry and exit portions relative to said
first flow direction of the reformate; the flow of the stream of
sweep gas through the permeate region in the second flow direction
is substantially the same as said first flow direction; and the
water gas shift catalyst is loaded substantially only in the entry
portion (74A) of the water gas shift membrane reactor.
Description
TECHNICAL FIELD
[0002] This invention relates to membrane water gas shift reactors
in a fuel processing system, and more particularly to membrane
water gas shift reactors included in a fuel processing system for
fuel cell power plants and the like.
BACKGROUND ART
[0003] There exists a need to provide hydrogen (H.sub.2) as a fuel
for various end uses, particularly as a fuel in fuel cell power
plants and the like. The hydrogen is typically chemically bound, as
in a raw hydrocarbon and/or including alcohol, or it may be in a
processed gas mixture such as syngas (H.sub.2 and CO), which, in
either event, is processed by a fuel processing system to provide a
hydrogen-rich fuel stream for eventual use as fuel for a fuel cell.
The raw fuel is typically reformed by a process that not only
provides a hydrogen-rich fuel stream, but which also results in the
production of carbon monoxide. Unfortunately, the carbon monoxide
is a very effective poison for low temperature fuel cells
(<100.degree. C.). The CO gets adsorbed on the noble metal
catalyst in the fuel cell stack, thereby preventing the H.sub.2
from reacting. Only a very small concentration of CO is necessary
to considerably reduce the number of the reaction sites available.
CO concentration of <50 ppm is typically required for a proper
operation of the fuel cell stack.
[0004] State of the art fuel processing systems rely on the
integration of several reactors and heat exchangers (HEXs), e.g., a
reformer, a water gas shift reactor train (WGS) and a preferential
oxidizer train (PROX) are thermally integrated to produce
reformate. This reformate can be fed into the stack after a final
CO cleaning is achieved by direct injection of oxygen into
reformate. The CO cleaning process (WGSs, PROXs and HEXs) adds
weight, volume, complexity and cost to the fuel cell power
plant.
[0005] Membrane reactors offer an inherent ability to combine
reaction, product concentration, and separation in a single unit. A
type of membrane reactor of particular interest is an integrated
water gas shift reactor with palladium alloy based membrane for
selectively removing hydrogen. Broadly speaking, a membrane reactor
includes a primary chamber or region containing a catalyst for
receiving a hydrogen-rich, gaseous derivative, e.g. reformate, of
the raw fuel and reacting the reformate to liberate hydrogen, a
secondary chamber or region for receiving nearly pure hydrogen as a
permeate from the first region, and a palladium membrane separating
the primary and secondary regions and providing a
hydrogen-selective permeability for the exclusive transfer of
hydrogen from the primary region to the secondary region.
[0006] Though membrane reactors have been discussed generally in
the literature, as for example in U.S. Pat. No. 6,228,147 to
Takahashi for operating a membrane reactor with a steam flow as a
sweep gas, relatively little or no discussion exists in the area of
the design of the membrane reactor and the integration of such a
system into a fuel cell-based power plant. For instance, a recent
U.S. Pat. No. 6,572,837 to Holland et al, though discussing a
hydrogen-separating membrane in use in a fuel processing system for
a fuel cell power plant, describes the hydrogen separation function
and structure as being physically separate from the various reactor
structures and functions.
[0007] An example of a fuel cell power plant that does incorporate
a water gas shift reactor integrated with a hydrogen-separating
membrane is illustrated and described in U.S. Pat. No. 6,423,435 to
Autenrieth, et al. That system tends to work at the relatively high
pressure, ie, greater than 10 bar, of 12 bar in the reformer and
WGS reactor, and then relies upon a relative vacuum of 0.5 bar to
remove the H.sub.2 from the hydrogen collecting space, but then
further re-pressurizes that H.sub.2 to about 1.5 bar to feed the
anode of the fuel cell. This system also employs some measure of
heat and water management to provide limited assistance to the
efficiency of the system. While the foregoing examples of fuel
processing systems each discuss various ways of separating hydrogen
from a reformate stream, at most only limited attention is given to
the efficient integration of that process into the overall system
of a power plant, particularly in a fuel cell power plant of the
type having a PEM (polymer electrolyte membrane) fuel cell assembly
that incorporates porous water transport plates and operates at or
near, ambient pressure.
[0008] It is an objective to provide an arrangement for enhancing
the efficiency with which hydrogen is removed from the reformate
stream of a fuel processor in the larger context of a power plant.
It is a further objective to do so through the use of a membrane
water gas shift reactor as part of a fuel processing system in the
context of a PEM-type fuel cell power plant. It is a still further
objective to do so through the use of a water gas shift reactor as
part of a fuel processing system in the context of a PEM-type fuel
cell power plant having a fuel cell assembly that incorporates
porous water transport plates and operates at moderate pressures
near or somewhat above ambient pressure.
DISCLOSURE OF INVENTION
[0009] The present invention pertains to a fuel cell power plant
system comprising at least one fuel cell stack assembly including
an anode, a cathode, and a coolant channel; a fuel processing
system for providing H.sub.2 to the anode and including a water gas
shift membrane reactor having a reaction region and a permeate
region separated by a H.sub.2 separation membrane, the reaction
region being connected to receive a supply of H.sub.2-rich
reformate and a supply of water for supporting a water gas shift
reaction of the reformate to enhance the production of H.sub.2 and
to shift CO to CO.sub.2, the produced H.sub.2 being selectively
separated from the reformate stream via the membrane to form a
permeate in the permeate region of the reactor, and the reformate
stream issuing from the reactor as a retentate; a source of heat;
and a water management system operatively connected to the fuel
cell assembly coolant channel for conducting water from and to the
fuel cell assembly. A stream of sweep gas is caused to flow through
the permeate region of the water gas shift membrane reactor to
facilitate the separation of H.sub.2 via the membrane. The water
management system is additionally connected to the source of heat
for converting some water to steam, and the steam may be
operatively connected to the permeate region of the water gas shift
membrane reactor to provide some or all of the sweep gas flowing
there through. In one embodiment, steam is the only sweep gas. In
another embodiment, steam is combined with an inert gas, such as
nitrogen (N.sub.2) from combusted or otherwise O.sub.2-depleted
air.
[0010] This integration of the power plant components and functions
yields a particularly efficient arrangement, from the standpoint of
overall plant efficiency, for generating H.sub.2 for use by the
fuel cell of the power plant.
[0011] The fuel cell stack assembly is of the PEM type (polymer
electrolyte membrane) that incorporates porous water transfer
plates (WTPs) for the efficient recovery of water from the fuel
cell stack assembly, which water is then available for use in the
processing of fuel into H.sub.2, and for the generation of the
steam for use as a sweep gas. The system also employs energy
recovery devices (ERDs) that serve to recover water from hot
exhaust streams while exchanging heat to incoming air.
Additionally, selective use of existing thermal energy sources,
such as the combustion of the retentate from the fuel processing
system for steam generation, enhances the efficiency of the
system.
[0012] In yet a further aspect of the invention, the sweep gas
stream is preferably caused to flow through the permeate region of
the water gas shift membrane reactor in a direction that is
counter-current (contra) to the flow of the reformate/retentate
stream to increase the efficiency for a given volume of the
membrane reactor.
[0013] In a still further aspect of the invention, a connected
combination of a compressor and an expander may be included
respectively before and after at least the membrane reactor to
efficiently provide limited pressurization of fluid flow through
the membrane reactor. That pressurization is moderate, typically
being in the range of 1 to 10 bars, and is preferably about 6 to 7
bars.
[0014] In yet a further aspect of the invention, the reaction
region of the water gas shift membrane reactor contains catalyst
loaded or arranged therein to yield an improved efficiency/cost
ratio. Particularly, assuming that the sweep gas flows in a
direction counter to the flow of the reformate/retentate stream, it
is beneficial to use catalyst at or near the opposite ends of the
reaction chamber, but to omit its use in the mid-region of the
reaction chamber where it is relatively ineffective. In the event
the sweep gas flows in a direction concurrent (cocurrent) with the
flow of the reformate/retentate stream, it is beneficial to use a
limited amount of catalyst only in the entry end or region of the
reaction chamber, but to omit its use from the mid-point to the end
of the reaction chamber where it is relatively ineffective.
[0015] The foregoing features and advantages of the present
invention will become more apparent in light of the following
detailed description of exemplary embodiments thereof as
illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0016] FIG. 1 is a schematic flow diagram of a fuel cell power
plant system employing a water gas shift membrane reactor in
accordance with the invention;
[0017] FIG. 2 is a perspective functional schematic illustration of
a water gas shift membrane reactor, including an expanded
longitudinal sectional view of a portion thereof, in accordance
with the invention;
[0018] FIG. 3 is a schematic flow diagram of part of an alternate
embodiment of a fuel cell power plant system employing a water gas
shift membrane reactor, depicting a sweep gas for H.sub.2 that
flows co-currently with the reformate/retentate flow; and
[0019] FIG. 4 is a schematic flow diagram of part of an alternate
embodiment of a fuel cell power plant system employing a water gas
shift membrane reactor, depicting the use of N.sub.2 as a sweep
gas
BEST MODE FOR CARRYING OUT THE INVENTION
[0020] Referring to FIG. 1, a fuel cell power plant 10 in
accordance with a preferred embodiment of the invention is
depicted, and includes a fuel cell assembly 12 of one or more fuel
cells, a fuel processing system generally designated 14, and a
coolant system generally designated 16. The fuel cell assembly 12
is preferably of the polymer electrolyte membrane (PEM) type and
includes anodes 18, cathodes 20, separating electrolyte membranes
22 and coolers 24 preferably formed of porous water transfer plates
(not separately shown) of a type described in U.S. Pat. No.
5,700,595 to Reiser and incorporated herein by reference. A
hydrogen-rich (H.sub.2) fuel, or permeate, stream 26 from the fuel
processing system (FPS) 14 is supplied to the anodes 18; a supply
of oxidant, such as air, is supplied via line 28 to the cathodes
20; and coolant in the form of water (H.sub.2O) circulates in the
coolant system 16, and particularly coolers 24, principally as a
product of the electrochemical reaction in the fuel cell assembly
12 and any additional make-up water that may be required. The
coolant water is additionally used for thermal transfer and/or as a
constituent of the reformation and/or WGS processes and/or as a
sweep gas in the FPS 14, as will be described.
[0021] In a coolant loop 16' that is local to the fuel cell
assembly 12, water exiting from coolers 24 is circulated by pump 30
through circuit 31 to a conventional degasifier/accumulator 32
where gas is removed from the coolant and the water is then
accumulated and available in liquid form for return to the fuel
cell assembly coolers 24 via line 33. Water accumulated in
degasifier/accumulator 32 is also available for use in the FPS 14
via line 16''.
[0022] The oxidant supplied to the cathodes 20 via line 28
preferably comprises the passage of ambient air through a gas
channel of a water transfer energy recovery device (ERD) 34 of
suitable known design, as for example of the type described in U.S.
Pat. No. 6,274,259 to Grasso et al and incorporated herein by
reference. The driver for that air flow may be, for example, a
blower 35. Spent oxidant laden with moisture is exhausted from the
cathodes 20 via the degasifier/accumulator 32 and thence on line 29
through the ERD 34, where it transfers heat and moisture to the
incoming air.
[0023] A stream of spent H.sub.2 is exhausted from the anodes 18
and is both recycled via line 36 by blower 37 to the inlet of the
anodes and is also conveyed via line 38 to a catalytic burner
40.
[0024] Having described the relatively conventional structure and
operation of the PEM fuel cell assembly 12, attention is turned to
a description of the FPS 14 and its integration with the fuel cell
assembly 12 and the coolant system 16. A supply of carbon-based
fuel 42, as for instance gasoline, natural gas or other similar
hydrocarbons, is delivered by fuel pump 43 via heat exchangers 44
and 45 where it receives heat, and line 46, to a hydrodesulfurizer
(HDS) 47 where sulfur is removed from the fuel. The HDS 47 may be
optional if desulfurized fuel is used. The desulfurized fuel is
then delivered via line 48 to an inlet region of a reformer 50,
which may be an autothermal reformer (ATR), a catalytic partial
oxidizer (CPOX), a catalytic steam reformer (CSR), or similar
reactor for the reformation of the fuel stock. For the reformation
process or reaction, the reformer 50 additionally typically
requires sources of oxidant (air) and water or steam.
[0025] A feedwater pump 51 in the water line 16'' delivers the
liquid water to various feeders to points of use in the FPS 14 as
will be described. Water from line 16'' is delivered to the inlet
region of the reformer 50 via feeder line 16a'' after receiving
some heat passing through the cold side of an anode precooler 52 in
line 16''.
[0026] Air for the reformer 50 is initially heated and humidified
by passage thru the ERD 34, and is then delivered via lines 54 and
54' by a driver, such as the gas compressor 53, through a heat
exchanger 56 where it receives heat indirectly from the exhaust of
the catalytic burner 40, and then to an inlet region of the
reformer via line 57. The compressor 53 is preferably paired with
and directly or indirectly driven by, a gas expander 55, the
functions of which will be described in greater detail hereinafter.
One or more water vapor injectors (sprays) 58 receive water from
line 16'' via feeder line 16b'', and are positioned and operative
to introduce water vapor to the air stream in line 57 prior to
and/or after passage through the heat exchanger 56. To the extent
the catalytic burner 40 may not be available as a source of heat,
as during system start-up, a separate, limited-capacity start
burner 59 is selectively and operatively connected (shown in broken
line) to the supply of fuel 42 and the inlet air supplied by the
ERD 34, such that the air may be warmed and supplied to line 57 for
delivery to the reformer 50. Similarly, a limited-capacity electric
heater may serve as the hot-side thermal source for the heat
exchanger 45 during start up, or as needed.
[0027] The reformer 50 operates to react the air, water and
carbon-based fuel in a well known manner to produce a stream of
reformate containing a mixture of H.sub.2, CO, CO.sub.2, H.sub.2O
(and N.sub.2). That reformate, after perhaps receiving a charge of
water vapor in a vaporizer section 49, issues from the reformer 50
via line 60 and passes through the hot side of a heat exchanger 61
and to the inlet region of a water gas shift (WGS) membrane reactor
62. The WGS membrane reactor 62 will be described in greater
detail, but suffice it to say at this point that the WGS reaction
on the reformate creates a gaseous mixture rich in H.sub.2 and in
which much of the CO has been desirably converted to CO.sub.2, then
most of the H.sub.2 is separated from the mixture via a separation
membrane 64, and the remaining constituents of the reformate stream
issue from the reactor 62 as an H.sub.2-depleted retentate stream
on line 66.
[0028] The retentate stream on line 66 continues to contain a small
amount of H.sub.2, and is supplied via expander 55 and line 66', as
one fuel source for the catalytic burner 40. Another fuel source
for that burner 40 is provided by unspent H.sub.2 in the anode
exhaust stream of line 38. Oxidant (air) for the combustion
reaction in burner 40 is supplied to the burner via blower 68. A
heated stream of air and combustion products exhausted from burner
40 is extended on line 69 through the heat exchanger 56, to provide
heat to the air being heated therein for supply on line 57 to the
reformer 50. After exiting the heat exchanger 56, the burner
exhaust stream 69 extends through the hot side of heat exchanger 44
and thence through the hot side 70A of a steam generating heat
exchanger 70A&B and is connected to the cathode exhaust line 29
prior to being exhausted through the hot/warm side of ERD 34. A
supplemental supply of heated water may be supplied as a spray to
the stream 69 by vapor injector 71 connected between heat exchanger
hot side 70A and the ERD 34, and receiving water from line 16'' via
feeder line 16c''. This serves to cool the exhaust, if needed, and
to add moisture to the stream that is recycled via the ERD 34.
[0029] System performance is enhanced by providing a portion of the
H.sub.2-containing reformate from the reformer 50, via line 60' and
through an ejector 72, to the HDS 47 for use in the desulfurization
process. The ejector is driven by a pressurized stream of water on
feeder line 16d'' connected to line 16 prior to precooler 52.
[0030] Attention is now turned to the WGS membrane reactor 62,
which forms a principal component of the invention. Referring to
FIG. 1 and additionally to FIG. 2, the WGS membrane reactor 62
comprises a WGS reaction region, generally designated 74,
containing an appropriate water gas shift catalyst 75, such as a
noble metal on an active support, or the like. The reaction region
74 may be comprised of an entry portion or section 74A, an exit
portion or section 74C, and an intermediate portion or section 74B
between the entry and exit portions or sections, for purposes to be
described below in greater detail. These sections are depicted for
ease of understanding the relative locations of shift catalyst 75
loaded in the WGS reaction region 74, and may vary in relative
size, etc. Moreover, the overall size and length of the WGS
reaction region 74 is a function of the catalyst activity and
positioning, as well as the desired rate of H.sub.2 separation.
Gaseous reformate on line 61 from reformer 50 is supplied to the
reaction region 74 where it undergoes the well-known WGS reaction
to convert much of the entering CO to CO.sub.2 and also further
increase the H.sub.2 available. This process includes the addition
of water, which may occur in a final stage of reformer 50, as
depicted in the present embodiment and represented by water vapor
injector 73 connected to water feeder line 16e'' to inject water
into the reformate in the vaporizer section 49. Alternatively, that
injection of water vapor may occur in a separate vaporizer unit
located between the reformer 50 and the WGS reactor 62, or it may
occur in the WGS reactor 62 itself. System performance is
additionally enhanced by recycling a portion of the retentate
stream on exit line 66 to the vaporizer section 49 by connection to
an ejector 63 which is driven by a pressurized stream of water
supplied by, for example, feeder line 16c''. This returns some of
the H.sub.2 remaining in the retentate, via line 66'', for recycled
reaction in the WGS membrane reactor 62.
[0031] To separate the H.sub.2 from the reaction products in the
reaction region 74 of the WGS membrane reactor 62, a membrane 64 of
H.sub.2-selective, permeable material forms an H.sub.2-permeable
boundary of the reaction region 74. The H.sub.2-selective material
is typically palladium or the like. Gaseous H.sub.2 that diffuses
through the H.sub.2-permeable membrane 64 accumulates as a permeate
in a permeate region 76, for transport to and use in, at least the
anodes 18 of the fuel cell assembly 12.
[0032] FIG. 2 depicts in somewhat greater structural detail, an
example of a WGS membrane reactor 62 in accordance with the
invention. The WGS reaction region 74 is formed within and defined
by a number of adjacent porous tubes depicted here as being formed
entirely of the membrane 64, though it will be understood that the
tubes may be a variety of materials and geometries, so long as the
H.sub.2 within the reaction region 74 is able to permeate through
the membrane portion 64 thereof to reach the permeate region 76. An
outer shell 77 loosely surrounds the several tubes comprising the
individual WGS reaction regions 74, such that the space or region
defined therebetween forms the permeate region 76 into which
hydrogen atoms diffusing through the H.sub.2-permeable membrane
flow and accumulate. From the permeate region 76, the H.sub.2 may
be delivered via line 26 and the warm side of anode precooler 52,
to the anodes 18.
[0033] An aspect of the invention is the reliance upon an
energy-efficient, intermediate-pressure regime for operation of the
fuel cell power plant 10. In that regard, it may be advantageous to
provide some pressurization in the FPS 14 to increase the pressure
differential across the membrane 64, yet also advantageous to limit
the size and capacity of equipment necessary to provide and contain
the resulting pressures. To that end, and in conjunction with the
PEM fuel cell assembly 12 that typically operates near ambient
pressure, the air on line 57 delivered to the reformer 50 is
pressurized in the range of 1-10 bar, and is preferably about 6
bar. This is accomplished by the compressor 53. Some of the
retentate exiting the WGS membrane reactor 62 on line 66 is fed to
the expander 55, where its pressure is reduced and then used to
partially fuel the catalytic burner 40. This expansion of the
retentate at the expander 55 also serves to recover energy which
may then be used to power a motor/generator 67 connected thereto,
and/or to drive the compressor 53 which is placed on the same shaft
65 as the expander, thus resulting in efficient energy usage.
Whereas the operating pressure in much of the FPS 14 is preferably
at an intermediate pressure of about 6 bar, it will be understood
that if operation at or near the ambient condition of 1 bar is
alternatively preferred, the need for the pressure controlling
equipment described above may be avoided.
[0034] At this juncture it is important to consider an important
aspect of the invention, that being the use of a sweep gas flowing
through permeate region 76 as not only facilitating transport of
the H.sub.2 to the anodes 18, but importantly also, as facilitating
the shifting of the WGS reaction towards the products within the
reaction region 74. This is accomplished by continued removal of
the H.sub.2 from the permeate region 76 so as to enable high
H.sub.2 partial pressure differentials across the membrane portion
64, which facilitates flow of H.sub.2 across the membrane portion
64 to decrease the H.sub.2 present in the reaction region 74, which
in turn acts to shift the WGS reaction equilibrium in a direction
favorable to the production of H.sub.2.
[0035] Although the sweep gas might be inert gas, nitrogen, the
H.sub.2-lean, moisture-laden exhaust from the anodes 18, or other
suitable fluids, including phase-change materials, the sweep gas,
designated 78 in FIG. 2, is preferably steam in accordance with the
preferred embodiment of the invention. The use of steam as the
sweep gas 78 serves to efficiently integrate and utilize the
thermal and coolant components existing in the fuel cell power
plant 10 and increase the life of the fuel cell assembly 12, by
extending the life of the polymer electrolyte membrane 22. The
steam forming the sweep gas 78 is provided by heating water in a
feeder line 16f'' that passes through the cool side 70B of steam
generating heat exchanger 70A&B and is connected to a
water/steam vapor injector 80. The heat is obtained from the burner
exhaust gas flowing in the hot side 70A of the heat exchanger. An
additional source of hot water supplied to vapor injector 80 is
obtained by extending a water feeder line 16g'' through the cool
side of heat exchanger 61 and thence to the injector 80. The heat
is obtained from the reformate flowing in line 60 through the hot
side of heat exchanger 61. In each instance, the water in line 16''
that enters feeder lines 16f'' and 16g'' would have been heated by
heat exchange with the H.sub.2 permeate in line 26 that passes
through the hot side of anode precooler 52 prior to delivery to
anodes 18. System efficiency is obtained by using these thermal
sources (burner exhaust and anode precooler) to provide the steam
for the sweep gas.
[0036] In accordance with yet another aspect of the preferred
embodiment of the invention, the sweep gas 78 in FIG. 2 is caused
to flow through the permeate region 76 in a direction that is
counter to (countercurrent) the direction in which the reformate
(represented by line and flow arrow 60) flows through the WGS
reaction region 74 and exits as retentate (represented by line and
flow arrow 66). In FIGS. 1 and 2, the flow of reformate is from
left to right, and the flow of the sweep gas is right to left
within the permeate region 76. Referring only to FIG. 1, the sweep
gas is not separately identified with a reference numeral, but may
be considered to enter the right side of permeate region 76 via the
vapor injector 80 and then flow leftward, entraining the H.sub.2
and becoming the permeate flow, or H.sub.2-rich fuel stream,
designated by line 26 flowing to the anodes 18. It has been found
that use of a counter-flow arrangement increases the efficiency for
a given volume of the membrane reactor 62.
[0037] Still further, it has been found that the catalyst 75, to
the extent it exists in the middle of the WGS reaction region 74,
here designated intermediate portion 74B, relative to longitudinal
flow of reformate therethrough, is not utilized effectively in a
counter flow configuration because the H.sub.2 and CO fractions are
too close to equilibrium to have appreciable affect on the rate of
the WGS reaction. However, towards the entry and exit end(s) or
portions 74A and 74C of the reaction region 74, as the hydrogen
partial pressure decreases, the shift reaction is promoted towards
the product side, such that having the catalyst at the end of the
reactor enables complete conversion of CO. Accordingly, in the
operational configuration where the sweep gas flows countercurrent
to the reformate flow, the tubes of membrane material 64 that
collectively define the WGS reaction region 74 are filed or
otherwise loaded with catalyst 75 at or near the entry portion 74A
and the exit portion 74C of the region/tubes, and the intermediate
portion remains relatively vacant for use only for hydrogen
separation. This loading is reflected in FIG. 1 by inclusion of the
word "Shift" and the catalyst reference numeral "75" only in the
entry and the exit sections of reactor 62. Depending upon catalyst
activity, the relative flow of sweep gas 78 and reformate, and the
extent of hydrogen removal, the loading of catalyst 75 might
typically be concentrated in approximately the first 20% and the
final 20% of the flow length of reactor 74, with little or no
catalyst loading in the intermediate section. Such a catalyst
loading profile would be appropriate for a 50 kW fuel cell based
power plant fueled by gasoline, for .about.78% hydrogen recovery,
and a 15 liter membrane reactor operating at .about.7 bars with a
membrane permeance of 30 m.sup.3/m.sup.2-hr-atm.sup.0.5, providing
that the permeance of the membrane is independent of the gas
composition. This configuration then affords a reduction in the
cost and amount of equipment and material required, while
preserving the efficiency of the system.
[0038] Referring to FIG. 3, there is depicted that portion of the
FPS 114 of a fuel cell power plant 110 that depicts an alternate
aspect of the invention. Because the components in the FIG. 3
embodiment are either the same or functionally similar to those in
FIGS. 1 and 2, the same numbering convention has been maintained,
however a "1" precedes those components that differ somewhat in
positioning or structure, and additional description or comment is
provided. The present embodiment differs mainly in that although
the steam providing the sweep gas on line 16f'' is developed in the
same way by heating in heat exchangers 52 and 70B and is injected
into the permeate region 76 by a vapor injector 180, it will be
noted that the injection occurs at the same end and direction of
the WGS membrane reactor 162 as the introduction of the reformate
on line 60. Thus, the steam sweep gas is directed to flow
cocurrently with the reformate stream. In this configuration, it
has been found that efficient usage of the catalyst 175 is
optimized if it is loaded mostly in only a limited portion of the
first half of the WGS reactor 174 (in the direction of reformate
flow), because it has been found to provide nearly the same results
as being loaded over the entire length. More specifically, the
majority of catalyst loading should occur over about 20% of the
first half of the WGS reactor 174, as represented by the catalyst
reference numeral 175 and the word "Shift" appearing in entry
portion 174A. While this configuration results in a reduction in
the amount of WGS catalyst used, it is limited because it is not
able to complete shift in the equilibrium due to accrued hydrogen
in the sweep gas at the exit of the reactor. This has the effect,
relative to the embodiments of FIGS. 1 and 2, of reducing the
driving force for hydrogen removal for practical operating
pressures.
[0039] Additionally in FIG. 3, it will be noted that the H.sub.2 in
retentate 66 that is recycled via the ejector 63 is conveyed via
line 66'' to a vaporizer 149 that is depicted as free-standing, or
separate, from a reformer or source of reformate, as was earlier
mentioned.
[0040] Referring to FIG. 4, there is depicted that portion of the
FPS 214 of a fuel cell power plant 210 that depicts an alternate
aspect of the invention with respect to the sweep gas 278. Because
most of the components in the FIG. 4 embodiment are either the same
or functionally similar to those in FIGS. 1 and 2, the same
numbering convention has been maintained, however a "2" precedes
those components that differ somewhat in positioning or structure,
new components are numbered beginning with a "2" and followed by
two digits in the 90's, and additional description or comment is
provided. This embodiment differs in that the sweep gas 278 is not
simply steam, but is an inert gas, such as nitrogen (N.sub.2) in
line 290, that is, or may be, conveniently accompanied by steam
16f''.
[0041] A second catalytic burner 292 receives H.sub.2 and water
from the anode exhaust via line 238, and an oxidant-depleted supply
of N.sub.2-rich air via line 229 from the exhaust of fuel cell
cathode 20. The burner 292 and the supply of N.sub.2-rich air are
regulated carefully to provide a gaseous exhaust stream that is
rich in N.sub.2 and substantially devoid of O.sub.2, and is
connected via line 290 to a vapor injector 280. Also connected to
the vapor injector 280 is the steam supply line 16f'', such that a
measure of steam (H.sub.2O) may be mixed with the N.sub.2 to
provide the resulting sweep gas 278. This particular arrangement
for using N.sub.2 as a significant portion of the sweep gas has the
advantages, relative to steam alone, that the vaporizer and heat
exchanger elements may be down-sized, and the use of steam is
decreased. While this arrangement may have the disadvantage of
slightly lowering the partial pressure of hydrogen supplied to the
fuel cell stack assembly 12 (due to the presence of gaseous
nitrogen in the fuel feed stream), it is a modest penalty.
[0042] Although the invention has been described and illustrated
with respect to the exemplary embodiments thereof, it should be
understood by those skilled in the art that the foregoing and
various other changes, omissions and additions may be made without
departing from the spirit and scope of the invention.
* * * * *